Charged particle energy analysis

ABSTRACT

Method and apparatus for energy analysis of a stream of moving charged particles by effecting electrostatic segregation of a particle portion having a preselected kinetic energy and thereafter counting the number of the particles.

nited States Patent [1 1 (IHARGED PARTICLE ENERGY ANALYSIS Jerald D. Lee, Wilmington, Del.

Assignee: E. 1. duPont de Nemours and Company, Wilmington, Del.

Filed: Aug. 3, 1971 Appl. No.: 168,575

Related US. Application Data Continuation-impart of Ser. No. 99,475, Dec. 18, 1970, abandoned.

lnventor:

us. Cl. 250/495 PE, 250/495 AE Im. cu. nol 37/26 Field 01 Search 250/495 A, 49.5 PE,

250/495 ED, 49.5 AE, 49.5 P

flehudug' Field H Pass 22b 1' I ill 21b I,

[56] Rellercnces Cited UNITED STATES PATENTS 3,500,042 3/1970 Castaing 250/495 P 3,579,270 5/1971 Daly 250/419 D Primary Examiner-James W. Lawrence Assistant ExaminerC. E. Church Attorney-Harry J. McCauley 57 ABSTRACT Method and apparatus for energy analysis of a stream of moving charged particles by effecting electrostatic segregation of a particle portion having a preselected kinetic energy and thereafter counting the number of the particles.

18 Claims, 9 Drawing Figures Filler PATENIEUJUI 31 um SHEEI 1 0f 4 fihoioelectrozp Peak zfyrmmd/ Energy Transmissiozz/ Current Election/Path mmvroe Jerald Dlee ATTORNEY PATENIEUJHI 3 I ma gag mVENme Jerald D. Lee

N QQN E ATTORNEY i CHARGED PARTICLE ENERGY ANALYSIS CROSS REFERENCE TO RELATED APPLICATIONS BRIEF SUMMARY OF THE INVENTION Generally, this invention comprises method and apparatus for energy analysis of a moving stream of electrically charged particles having different kinetic energies comprising, in seriatim, constraining a stream of charged particles having diverse kinetic energies to a given flow path, segregating and altering the direction of travel of a first fraction consisting of substantially all of the charged particles having kinetic energies below a preselected high energy level, directing the first fraction of charged particles to a spherical grid retarding potential filter maintained at a preselected electrical potential level barring forward longitudinal passage through the flow director of substantially all of the charged particles having kinetic energies below a preselected low energy level while permitting the forward passage of a second fraction of the charged particles constituting the remainder of the first fraction of the charged particles, and determining the number of charged particles constituting the second fraction.

DRAWINGS The following drawings depict preferred embodiments of the invention as applied to electron spectroscopy in which:

FIG. 1A is a plot of transmission current v. energy for the photoelectron stream emitted when a sample is bombarded with X-rays,

FIG. 1B is a plot of transmission current v. energy characteristic of a narrow-band energy-pass filter at an arbitrary energy E.,,

FIG. 2 is a schematic side elevation cross-sectional view of a typical prior art hemispherical electrostatic analyzer,

FIG. 3A is a partially schematic longitudinal sectional view of a first embodiment of apparatus according to this invention,

FIG. 3B is an exploded perspective view of the apparatus of FIG. 3A,

FIG. 3C is a partially schematic fragmentary longitu dinal elevation sectional view of the right-hand, charged particle reversal end of a second embodiment of apparatus according to this invention,

FIG. 4 is a diagrammatic representation of the acceptance angle for the apparatus of FIGS. 3A and 3B,

FIG. 5 is a diagrammatic representation of'the cooperative relationship of the successive prefiltering, low pass filtering and high pass filtering utilized in the operation of this invention, and

FIG. 6 is a typical spectrum record obtained in the analysis of a gold sample using apparatus constructed according to this invention.

GENERAL Charged particle analysis according to this invention can be applied to a wide variety of elemental charged particles, such as alpha and beta radiation, ions and the like; however, it is particularly useful in the conduct of electron spectroscopy and, accordingly, is hereinafter described in particular application to this apparatus and technique.

Electron spectroscopy for chemical analysis is a comparatively new procedure which has been described extensively in the article entitled Electron Spectroscopy for Chemical Analysis (ESCA) by Kai Siegbahn et al.,

.Uppsala University, Uppsala, Sweden (1968) October (Processed for the Defense Supply Agency by the Clearing-house For Scientific and Technical Information with the identification number AD-844-315).

Briefly, electron spectroscopy is the study of the energy (velocity) distribution of secondary electrons (photoelectrons) emitted by a sample upon irradiation of the sample by a primary energy source, such as a beam of X-rays. The operation is conducted by an electron spectrometer having a radiation source for exciting a sample, means for analyzing the velocities (energies) of the secondary electrons released due to the excitation, and means for recording electron energy vs. the quantity (current) of electrons falling within small increments of energy. The apparatus utilizes high vacuum pumps, a high-voltage source, an X-ray or other emitter of exciting energy, a sample module or holder, an energy analyzer, and a readout device such as an X-Y recorder.

ESCA has broad application to the analysis of the full range of individual chemical elements, even in the presence of other elements, and is particularly effective in organic chemistry, since the chief constituent elements carbon, nitrogen, oxygen, etc., are relatively easy to study. In addition, electron spectroscopy is better suited than X-ray analysis for studying the atomic structure of surfaces, because the secondary electrons (as contrasted with secondary X-rays), are emitted only from a surface layer A or less in thickness. Thus, information on composition, bonding states and the like peculiar to the surface exclusively is readily obtainable using this tool.

When a sample under analysis is irradiated from a primary source the sample emits photoelectrons in essentially random directions and at velocities (energies) unique to the specific electron-level structure of the atoms in the sample. To be of value in chemical analysis, these photoelectrons must be categorized with respect to their energies and the number of electrons emitted in each energy category determined over a given interval of time. This categorizing is effected by an energy analyzer, such as the design provided by this invention.

A successful analyzer must (a) provide high resolution, i.e., separation of electron fractions of closely adjacent energies and (b) provide high sensitivity, i.e., measureable and representative readouts for each small energy increment. Also, the analyzer must accommodate a high electron throughput, or luminosity, so that electron energy categorization can be accomplished within a relatively short time interval.

The energy distribution spectra of photoelectrons produced by X-ray excitation is such that the electrons which characterize a specific element lie at particular energy levels and, in general, are manifested as discrete maxima resting upon a background having a broad distribution of energy. This background exists because electrons which would otherwise have discrete energies which characterize the element (or sample) have undergone collisions within the sample and thus have lost varying amounts of energy. Another source of background current is the exciting X-ray background (bremsstrahlung), which is superimposed upon the desired exciting X-rays of narrow energy distribution (characteristic X-rays). A typical energy distribution of electrons emitted by a sample, including the unavoidable background, is represented by FIG. 1A.

Since it is desired to measure only the intensity and shape ofthe photoelectron peak representing a discrete energy difference, both the sensitivitY and accuracy of measurement are reduced proportionately if any of the background other than that directly under the peak is measured. This occurs because the statistical variation in the number of electrons arising from the broad background is large. For this reason, it is preferred to measure the current (representative of the number of electrons in that narrow energy band) within a photoelectron peak with a narrow-band energy pass filter. FIG. 1B shows the desired transmission characteristics of such a narrow-band energy pass filter at an arbitrary energy E,,. One such narrow-band pass filter of the prior art which accomplishes this type of discrimination is a hemispherical electrostatic analyzer, illustrated schematically in side elevation cross-section in FIG. 2. Only those electrons with energy E eeR,,/2 can pass both slits, where e is the electric field produced by the potential difference V, e is the charge on an electron and R is the radius of the hemisphere including the slits.

DETAILED DESCRIPTION This invention relates to an improved apparatus and method for performing an energy analysis on charged particles. The example chosen in detailing operation involves the secondary electrons produced in an X-ray photoelectron spectrometer. The instant method of energy analysis employs a novel arrangement of electron deflecting and retarding elements, including a spherical electron mirror which, in combination with associated electrostatic lens elements, produces a many-fold increase in electron counting rate while retaining entirely satisfactory resolution preservation.

Applicants parent Application Ser. No. 99,475 supra described an electrostatic analyzer for use in X-ray photoelectron spectroscopy particularly which employed a double-filter type of electron energy analysis by which a narrow band-pass energy selection was achieved by causing the electron beam to encounter, in succession, a high energy pass filter, followed by a low energy pass filter, both filter devices being essentially of the coaxial cylindrical variety. In contrast, the apparatus of the instant invention, while still functioning as a narrow band pass analyzer, comprises a low energy pass filter in the form of an electron mirror, followed by a high energy pass filter of the spherical grid design. Elimination of the high-energy electrons in the initial electron beam by a low-pass parallel-plate filter, or other means, is resorted to in both inventions. However, a remarkable improvement in sensitivity, by at least an order of magnitude, is obtained by the instant invention over the apparatus of Application Ser. No. 99,475.

Accordingly it is an object of the present invention to provide an energy analyzer apparatus for use in X-ray photoelectron spectroscopy and the like which pos- 6 sesses an enhanced analysis sensitivity, while permitting a corresponding reduction in X-ray source intensity and X-ray power requirements.

Other objects of the present invention are:

a. the provision of an improved narrow bandpass energy analyzer having improved spectral line determination by the obtainment of a higher signal-tonoise ratio than achieved by the prior art,

b. the provision of an improved energy analyzer affording, for a given resolving power, a larger effective entrance aperture and acceptance angle, thereby achieving a high electron count rate, and

c. the provision of an improved design of energy analyzer simple in construction, versatile in use and reduced in cost.

The following fundamental relationships are applicable to analyzer performance:

The current, i, in monoenergetic electrons per second counted at the exit slit of any spectrometer is given by the following equation:

A/E Anf, or i= BAQf,

where 8,, brightness of photoelectrons ejected from the sample by the X-ray source. Its dimensions are number of electrons/(unit area of samples.) (unit solid angle.) (second) (E /E the ratio of the electron energy in the analyzer to the energy of the electron when ejected from the sample. The velocity (energy) of the electrons is often deliberately retarded prior to analysis to obtain better resolution, AE, where AB E -15,, as to which E, is the high energy level whereas E is the low energy level of the secondary electron fraction analyzed,

AE/E is a constant of the spectrometer instrument;

hence better resolution AE can be obtained by operating the analyzer at reduced E e.g., by applying a retarding field between the sample and the entrance slit of the spectrometer,

A area of entrance aperture of energy analyzer,

Q acceptance angle of energy analyzer in steradians, where a steradian is a unit solid angle,

f= efficiency of analyzer (sometimes called 37 transmission) in passing electrons in the desired energy range. In the instant invention it is less than unity, because of the interpositioning of absorbing screens across the paths of the electrons, B B /E E, brightness at the entrance aperture to the spectrometer.

Sometimes the quantity Aflf is called the "luminosity" of the analyzer and is abbreviated L.

Equation (1) gives the count rate i for the case in which the resolution of the analyzer, AE, is at least as broad as the spectral line. If the resolution is less than the spectral line width, Eq. (1) must be modified to:

where B,,= B /unit energy Analogously, L then becomes spectral luminosity A Of AE.

The count rate decreases at higher resolution (smaller AE) because higher resolution is obtained by decreasing one or more of the following factors:

a. entrance slit area A b. collection angle c. analyzer energy E,,. g

In the instant invention, and that of Application Ser. No. 99,475 also, enhanced resolution is obtained by decreasing E It is a known law of electron optics that increased retardation to decrease E is accompanied by a decrease in brightness B at the entrance aperture to the spectrometer, contributing to decreased electron transmission. However, this loss in brightness can be compensated for by increasing the spectrometer entrance aperture A and acceptance angle 0, as is clear from Equations (1) and (2) supra. It will be seen that this is accomplished in the instant invention by the use of a spherical electron mirroror other means, which performs, simultaneously, a focusing as well as an electron monochromatization step.

In principle, an electron mirror is the analog of its light-optical counterpart, except that reflection in the former does not occur at a single surface but is distributed through a region, depending upon the gradation of equipotential surfaces as produced by a specific arrangement of electrodes and potentials. Electrons incident on the mirror are slowed down as they penetrate into the mirror field until they reach the zero equipotential, at which point their velocity vanishes and their direction of motion is substantially reversed. Thus, electron mirrors can perform a dual role. First, they image the object projected on the mirror and, second, they control the intensity (and direction) of the electron beam. Such mirrors can thus act as a low-pass energy filter, i.e., the mirror eliminates electrons which are incident with an initial energy greater than a predetermined value (cut-off energy) and reflects back the remaining electrons of lower energy. These lower energy electrons can then be focused by the mirror for further energy analysis.

When an electron mirror is used as an energyanalysis element in an electron spectrometer as taught in this invention, it can be shown that the resolving power ef the spectrometer is, in large part, determined by the solid angle, d0, drawn from the mirror and subtended by the entrance aperture A of area s (refer FIG. 4), in accordance with the relationship AE/E =*-4(d0).

At the same time, the size of the acceptance angle Q is in part determinative of electron counting rate, as can be seen from Equation (1) supra. Given a desired resolution AE/E, then, the mirror parameters E, and Q can be specified such that a given electron counting rate can be attained, since count rate is proportional to luminosity (L), i.e.,

A s (d0)'R and L A 0.

Thus, the importance of a large entrance aperture A and a large acceptance angle Q can be clearly seen. In the present invention, a concave spherical electron mirror ideally provides for selection of the optimum initial values of these parameters. Moreover, the mirror can be used in conjunction with other electron optical elements, such as lenses, to preserve this advantageous selection while performing a narrow-band spectral determination.

The method of my invention will be more clearly understood by reference to FIG. 5, which is a plot of electron transmission, or count, versus electron energy. The solid line, denoted low-pass, depicts the characteristic filtration of electrons emanating from the X-ray source by an electron mirror so placed as to intercept a large percentage of the incoming electrons. Proceeding from left to right along this curve, it is seen that, at a given energy level M, there 'is a relatively sharp cutoff of electrons passed (i.e., reflected) by the mirror, all electrons above this level being eliminated from further analysis. Thus, the area under this curve represents the total number of electrons in the lower energy rangethat are passed for further analysis. Of these, the only electrons of interest are those at the higher end of this low-energy band. These are the electrons which fall within the region of common overlap of the solid curve denoted high-pass with that of the low-pass curve. In the instant invention it will be hereinafter seen that this high pass filtering action is imposed by a spherical grid arrangement providing a retarding potential. Superimposed on the plot of FIG. 5 are two other curves, one in broken line representation, denoted prefilter, the significance of which will be made clear in the subsequent description.

Referring now to FIGS. 3A and 38 there is shown, respectively, in schematic side-elevation section and in exploded perspective views, a preferred embodiment of electron energy analyzer according to this invention.

It will be understood that the entire analyzer shown in FIG. 3A is housed within an evacuated enclosure (not shown). The exciting X-rays constitute a substantially monochromatic beam of radiation which is passed, along line N, through an opening in tubular element 10 closed off with a I foil aluminum filter, so that it impinges on the sample 11 to be analyzed at an angle of approximately 20 therewith, causing the emission of photoelectrons from sample ll.

Measurement of the binding energy of the photoelectrons emitted from the surface of sample 11 enables a semi-quantitative analysis of the surface to be made. In the instant invention, this measurement is made as follows: the beam of photoelectrons leaving sample 11 passes axially along line 112 through tube 10 and frustoconical tubular element 13, communicating therewith, which subtends a relatively large solid angle around sample 11. It then passes through electron condensinglens element 14 interposed between tubular elements 13 and 15.

Truncated tubular element extension 15a forms a juncture with the upper electrode ll7a of the sloped parallel plate assembly denoted generally at 17 (FIG. 3B) as well as with the main tubular enclosure wall denoted T. There is imposed a variable retarding field between tubular elements 13 and 15. The electron beam 12 is focused by lens element 14 to a central spot at the opening 16a, which is a screen-grid aperture.

Parallel plate assembly 17 constitutes a low-pass prefilter for the photoelectrons entering opening 16a and comprises, in addition to deflecting plates 17a and 17b, an electron absorption and ground plate 176, seen to somewhat better advantage in FIG. 3B. Prefilter 17 also incorporates two verticallyoriented parallel plates 17d and ll7e, making the assembly a double focusing subassembly. The function of a prefilter of the design described is taught in applicant's copending Application Ser. No. 99,475 supra, and will be discussed in further detail hereinafter with respect to the instant invention.

Electrons below the cut-off energy limit of prefilter 17 are deflected through an angle of approximately and thence pass to the central area of the solid-backed concave spherical electron mirror 1%, as indicated by the continuation of line 12. Screen-grid aperture 16b serves as an entrance aperture (presenting an opening of, typically, 7% inch dia. measured along the width of plate 17a, the projection of which in a plane normal to longitudinal axis l-I measures about V4 inch high) for the beam passing to electron mirror 18, and is disposed slightly above axis H.

The electron beam is redirected from mirror 18 in rays as indicated generally by line 19 through the lower half of centrally-apertured disc electrode 20, maintained at ground potential, at which point a real image of prefilter aperture 16b is formed.

The electron beam then passes to a high-pass filter means of conventional double spherical grid construction, denoted generallyat 21, adjoining electron collection means 22. Electron collection means 22 comprises an open cylindrical portion 22a constricted at the outboard end to form a centrally-apertured beam exit 22b in prolongation with which is mounted a conventional channel electron multiplier 23.

The tubular portion comprising collector 22 and spherical grid assembly 21 is inclined upwardly at a slight angle, typically, O.7, with respect to axis H to the left of vertical F-F, FIG. 3A, in order to yield better resolution. This tilting allows the collector axis to coincide with the geometric axis of the lower half of aperture 20a in disc electrode 20. The same effect can be accomplished by a downward translation of the high-pass filter.

Between prefilter 17 and electron mirror 18 there is interposed a conventional quadrupole lens 24 made up on a pair of vertical arcuate plates 24a (refer FIG. 3B) and a pair of horizontal arcuate plates 24b, which serve to position the reflected electron beam in the lower half of aperture 20a. A spherical grid 25, maintained at ground potential, is positioned between the quadrupole lens 24 and mirror 18, comprising a portion of the lowpass filter means associated with the mirror.

Ring electrodes 26 and the spherical mirror 18 are maintained at different negative potentials with respect to grid 25 increasing in negative magnitude in the direction of electron beam movement toward mirror 18. Segmentation of the electrostatic metallic surfaces in this manner in the region between grid 25 and mirror 18 aids in preventing fringing fields, thereby contributing to the attainment of high resolution. Similarly, there is incorporated between spherical grids 21a and 21b (FIG. 38) a plurality of separated ring electrodes 21c maintained at increasing negative potential in the direcion of travel of reflected beam 19, which, in combination with grids 21a and 21b, function to retard the reflected electron beam to an extent such that only those electrons reflected by mirror 18 having an energy greater than a predetermined cut-off value will surmount the high-pass barrier and move on to the collector. Thus, a narrow-band energy selection is made.

Dimensionally, the electron analyzer of FIGS. 3A and 3B is constructed to provide, typically an acceptance angle .Q(see also FIG. 4) of about rs steradian between the aperture 16b of prefilter l7 and the mirror surface. This is accomplished by providing a mirror radius of 5 inch measured from the lowest point d of aperture 16b to grid 25, with the chord of grid 25 measured normal to axis H being 2 inch. At least this same acceptance angle is provided by frusto-conical tubular element 13, such that mirror 18 enables the relatively large solid angle at the sample to be preserved throughout the entire folded length of the electron beam.

Typical potentials maintained on the several components of the analyzer during operation are as follows:

equally distributed between Ring electrodes 21c 0 and 74.9 volts Grid 21b 0 volts Grid 21:: 74.9 volts Collector 221: 74.9 volts Other elements at zero volt potential are the channel electron multiplier 23 and the circularly apertured diaphragm electrode 20, as well as the entire tubular enclosure body T between spherical grids 21b and 25. All lens electrodes are supported in, and insulated from, tube T by polytetrafluoroethylene insulators.

Collector means 22 receives the decelerated electrons passed by the double-grid assembly 21. A negative potential bias of, typically, 74.9 volts is maintained on the inner wall 22a of cylindrical cavity 22, causing the electrons entering cavity 22 to be contained therein and drawn to channel electron multiplier 23 maintained at zero potential. The collector 22 is essentially field-free, with the exception of the attractive influence of the channel electron multiplier. A typical path followed by a photoelectron in collector 22 is shown at R.

Typical spacing between parallel plate electrodes 17a and 17b is 0.35 inch, this value being determinative of the divergence in the electron beam passed to mirror 18 by prefilter 17. The distance from screen aperture 16a to the sample surface is, typically, 3 V4 inch. Centrally apertured disc electrode 20 aids in the rejection of electrons having energies above the desired narrow pass energy band as well as in the rejection of any secondary parasitic electrons scattered from mirror 18. The bottom half of this central aperture 20a serves as an entrance aperture for filter means 21 and serves to determine the resolution thereof.

Plate 17c, at ground potential, serves to isolate the reflected electron beam from the field of prefilter 17. At the same time, the electrostatic bar construction 17f of plate 17b improves the field planarity in prefilter 17 having a potential distribution ranging from zero to the potential of 17b.

It is an important feature of this invention that electron mirror 18 is preceded by low-pass filter 17. Otherwise, high energy electrons (e.g., those striking solidbacked mirror 18 with I00 volts or more energy) would cause the production of copious low energy parasitic secondary electrons which would give a pseudoreflection effect as manifested by solid line P in FIG. 5. Use of prefilter 17, having the broken line characteristic plotted in FIG. 5, attenuates this effect over the energy range of interest.

In operating the analyzer to take a spectrum, the narrow energy pass band, as determined by the respective cut-off values of low-pass spherical mirror filter l8 and high-pass spherical grid filter 21, is held constant and scanning is accomplished by changing the potential at ponint A or equivalently decelerating the beam at any point prior to aperture 16a.

By comparison with the analyzer of my Application Ser. No. 99,475 supra, the analyzer of the instant invention makes possible an increase in resolution from 1.8ev to l.2ev, and a simultaneous increase in sensitivity from about 40 counts/ma to about 1600 counts/ma. of X-ray current.

-By way of illustration, FIG. 6 shows a spectrum of the gold doublet taken by my instant apparatus. Aluminum Ka irradiation of the sample was used. The electron lines of gold, N and N are shown. At the peak level of the N line, the electron count rate was 49,000 counts/see, with a resolution at half height of l.2ev for this line. The X-ray power used to produce this spectrum was 10Kv at 30 ma. Electron mirror 18 was operated at 67.5 volts as was also the highpass retarding grid system. At the same resolution, a count rate of less than 10,000 counts/sec. would be achieved with the analyzer of Appln Ser. No. 99,475 for the same line, operating at l0 Kv. and 190 ma. The improvement obtained with the instant analyzer is, no doubt, in large part due to the large acceptance angle 0 which, at the uppermost level, could probably be further extended to about one steradian.

To obtain equivalent performance from the Appln Ser. No. 99,475 analyzer, a scale-up in size of approximately 5.5 times would be necessary, which would im pose additional space requirements and high-vacuum maintenance problems. Thus, one advantageous feature of the instant invention is the reduced size of apparatus coupled with improved performance.

The analyzer of this invention can be constructed of a variety of metallic materials. Typically, tubes 10, 13 and 15, 15a can be of aluminum. Mesh lens 14 can be of copper. Plates 17a, b, c, d and e can be of stainless steel. Bars 17fcan be of copper. Circular disc electrode 20 can be aluminum. Quadrupole lens 24 can be brass. Hemispherical grid 25 can have an aluminum ring and a mesh construction of steel or copper. Ring electrodes 26 can be brass. Spherical solid-backed mirror 18 can be aluminum. Collector 22 can be stainless steel. Grids 21a and 21b can have a mesh of steel and copper, or steel and nickel, with aluminum rims.

The mesh size of screen 21a is 100 lines/in, affording 80 percent transmission to the electron beam, i.e., 20 percent is wire area and 80 percent is area of apertures. All other screens have a mesh size of 20 lines/inch and 97 percent transmission. The purpose of the fine mesh, held at the desired potential, is to present, as nearly as possible, a ripple-free equipotential to the electrons reaching the respective filters. However, even with the finest available mesh, the defining equipotential will never be perfectly ripple-free, since the mesh itself will impart low voltage ripples.

A second embodiment of apparatus according to this invention is shown in FIG. 3C, wherein 18 is an openmesh spherical electron mirror substituted for the solid-backed mirror 18 of the embodiment of FIGS. 3A and 38. Mirror 18' is preceded by identical spherical grid 25 and ring electrodes 26 as in the first embodiment hereinbefore described and, in fact, mirror 18' can be identical in construction with screen 21a, i.e., having a mesh size of 100 lines/in, providing 20 percent wire area and 80 percent aperture area.

In this construction most of the high energy electrons pass through the meshes of the mirror without striking the wires, so that there is no spurious secondary electron generation added to the sample radiation. In order to trap high speed electrons traversing mirror 18' it is preferred to provide an electron trap 30 just past the mirror, and this can simply comprise a honeycomb metal structure such as utilized in an automobile radiator, closed on the back side to bar passage of electrons therethrough and maintained at a positive potential of, typically, =l00 to =300v to securely retain all electrons clearing the mirror.

It is usually preferable to use a prefilter 17 in conjunction with an open-mesh spherical mirror 18 but, in some instances, mirror 18 can be used alone.

It is preferred to use a mesh of maximum open area and, if desired, a pair of planar mirrors can be substituted for mirror 18, in which case these should be preceded by an electronstatic collimating lens which will then focus electrons returning along path 19, thereby effecting the same focusing function otherwise contributed by mirror 18.

What is claimed is:

1. An apparatus for analyzing the energy distribution of electrically charged particles having different kinetic energies, comprising, in combination:

a. a source means for introducing charged particles into the analysis section of said apparatus;

b. a low pass filter which reflects and focuses a first fraction of said charged particles, said first fraction consisting of those charged particles incident on said low pass filter from said source means having kinetic energies below a preselected high level energy limit;

c. a retarding field high pass filter maintained at a preselected electrical potential level barring passage of the charged particles within said first fraction having kinetic energies below a preselected low level energy limit while permitting passage of a second fraction of said charged particles constituting the remainder of the charged particles in said first fraction;

blocking means located between said low pass filter and said retarding field high pass filter, said blocking means being disposed relative to the focus of said low pass filter to allow passage of said first fraction to said retarding field high pass filter while preventing passage of substantially all other charged particles, and relative to said retarding field high pass filter to serve as an inlet means from which said first fraction of charged particles diverge and pass to said retarding field high pass filter which is constructed so that the equipotential surfaces generated by said high pass filter are substantially normal to the trajectories of the particles within said first fraction at their point of incidence on said high pass filter; and

e. means for detennining the number of charged particles constituting said second fraction.

2. The apparatus of claim 1 wherein said low pass filter is a spherical low pass filter and said retarding field high pass filter is a spherical-grid retarding potential filter.

3. The apparatus of claim 2 wherein said blocking means is an aperture means.

4. The apparatus of claim 3 wherein said low pass filter comprises an open mesh electron mirror followed by an electron trap for isolating substantially all of the charged particles incident on said low pass filter having kinetic energies above said preselected high level energy limit.

5. The apparatus of claim 3 further comprising an electrostatically charged prefilter maintained at a second preselected electrical potential level above that at which said low pass filter is maintained for preventing substantially all of the charged particles emanating from said source means with energies above said second preselected electrical potential level from passing to said low pass filter.

6. The apparatus of claim 5 wherein said low pass filter comprises a solid backed electron mirror.

7. The apparatus of claim 5 wherein said low pass filter comprises an open mesh electron mirror.

8. The apparatus of claim 7 wherein said open mesh electron mirror is followed by an electron trap for isolating substantially all of the charged particles incident of said low pass filter having kinetic energies above said preselected high energy limit.

9. The apparatus of claim 5 further comprising an electron condensing lens located between said source means and said prefilter.

10. The apparatus of claim 5 wherein said prefilter comprises a parallel plate double focusing subassembly.

11. The apparatus of claim 3 wherein said means for determining the number of charged particles constituting said second fraction is an electron multiplier.

12. The apparatus of claim 3 further comprising a conductive tube shielding the region between said low pass filter and said spherical-grid retarding potential filter.

13. An energy analyzer for a moving stream of electrically charged particles having different kinetic energies, comprising, in combination:

a charged particle inlet means;

b. a low pass filter which reflects and focuses a first fraction of said charged particles, said first fraction consisting of those charged particles incident on said low pass filter from said charged particle inlet means having kinetic energies below a preselected high level energy limit;

0. a spherical-grid retarding potential filter maintained at a preselected electrical potential level barring passage of the charged particles within said first fraction having kinetic energies below a preselected low level energy limit while permitting passage of a second fraction of said charged particles constituting the remainder of the charged particles in said first fraction;

d. blocking means located between said low pass filter and said spherical-grid retarding potential filter, said blocking means being disposed relative to the focus of said low pass filter to allow passage of said first fraction to said spherical-grid retarding potential filter while preventing passage of substantially all other charged particles and relative to said spherical-grid retarding potential filter to serve as an inlet means for said first fraction to said spherical-grid retarding potential filter; and 5 e. means for determining the number of charged particles constituting said second fraction.

14. The energy analyzer of claim 13 wherein said low pass filter is a solid backed electron mirror.

15. The apparatus of claim 14 further comprising an electrostatically charged prefilter maintained at a second preselected electrical potential level above that at which said low pass filter is maintained for preventing substantially all of the charged particles emanating from said charged particle inlet means with energies above said second preselected electrical potential level from passing to said low pass filter.

16. A method for analyzing the energy distribution of electrically charged particles emanating from a source with different kinetic energies, comprising the steps of:

a reflecting and focusing a firstv fraction of said charged particles consisting of substantially all of those charged particles incident on a low pass refleeting and focusing filter from such source with kinetic energies below a preselected high level energy limit;

b. directing said first fraction of charged particles to a blocking means disposed relative to the focus of the low pass filter to allow passage of said first fraction of charged particles while preventing passage of substantially all other charged particles;

c. further directing said first fraction of charged particles which diverge from said blocking means to a retarding field high pass filter which is constructed so that the equipotential surfaces generated by said high pass filter are substantially normal to the trajectories of the particles within said first fraction at their point of incidence on said high pass filter and which is maintained at a preselected electrical potential level barring passage of those charged particles within said first fraction having kinetic energies below a preselected low leVel energy limit, while permitting passage of a second fraction of charged particles constituting the remainder of the charged particles in said first fraction; and

d. determining the number of charged particles constituting said second fraction.

17. The method of claim 16 wherein said charged particles are subjected to preliminary concentration by passage through an electron condensing lens.

18. The method of claim 16 wherein said charged particles are subjected to prefiltration, prior to incidence on said low pass filter, to substantially eliminate the effect of parasitic secondary electrons resulting from the impingement of high energy electrons on said electron mirror.

* i t l 

1. An apparatus for analyzing the energy distribution of electrically charged particles having different kinetic energies, comprising, in combination: a. a source means for introducing charged particles into the analysis section of said apparatus; b. a low pass filter which reflects and focuses a first fraction of said charged particles, said first fraction consisting of those charged particles incident on said low pass filter from said source means having kinetic energies below a preselected high level energy limit; c. a retarding field high pass filter maintained at a preselected electrical potential level barring passage of the charged particles within said first fraction having kinetic energies below a preselected low level energy limit while permitting passage of a second fraction of said charged particles constituting the remainder of the charged particles in said first fraction; blocking means located between said low pass filter and said retarding field high pass filter, said blocking means being disposed relative to the focus of said low pass filter to allow passage of said first fraction to said retarding field high pass filter while preventing passage of substantially all other charged particles, and relative to said retarding field high pass filter to serve as an inlet means from which said first fraction of charged particles diverge and pass to said retarding field high pass filter which is constructed so that the equipotential surfaces generated by said high pass filter are substantially normal to the trajectories of the particles within said first fraction at their point of incidence on said high pass filter; and e. means for determining the number of charged particles constituting said second fraction.
 2. The apparatus of claim 1 wherein said low pass filter is a spherical low pass filter and said retarding field high pass filter is a spherical-grid retarding potential filter.
 3. The apparatus of claim 2 wherein said blocking means is an aperture means.
 4. The apparatus of claim 3 wherein said low pass filter comprises an open mesh electron mirror followed by an electron trap for isolating substantially all of the charged particles incident on said low pass filter having kinetic energies above said preselected high level energy limit.
 5. The apparatus of claim 3 further comprising an electrostatically charged prefilter maintained at a second preselected electrical potential level above that at which said low pass filter is maintained for preventing substantially all of The charged particles emanating from said source means with energies above said second preselected electrical potential level from passing to said low pass filter.
 6. The apparatus of claim 5 wherein said low pass filter comprises a solid backed electron mirror.
 7. The apparatus of claim 5 wherein said low pass filter comprises an open mesh electron mirror.
 8. The apparatus of claim 7 wherein said open mesh electron mirror is followed by an electron trap for isolating substantially all of the charged particles incident of said low pass filter having kinetic energies above said preselected high energy limit.
 9. The apparatus of claim 5 further comprising an electron condensing lens located between said source means and said prefilter.
 10. The apparatus of claim 5 wherein said prefilter comprises a parallel plate double focusing subassembly.
 11. The apparatus of claim 3 wherein said means for determining the number of charged particles constituting said second fraction is an electron multiplier.
 12. The apparatus of claim 3 further comprising a conductive tube shielding the region between said low pass filter and said spherical-grid retarding potential filter.
 13. An energy analyzer for a moving stream of electrically charged particles having different kinetic energies, comprising, in combination: a charged particle inlet means; b. a low pass filter which reflects and focuses a first fraction of said charged particles, said first fraction consisting of those charged particles incident on said low pass filter from said charged particle inlet means having kinetic energies below a preselected high level energy limit; c. a spherical-grid retarding potential filter maintained at a preselected electrical potential level barring passage of the charged particles within said first fraction having kinetic energies below a preselected low level energy limit while permitting passage of a second fraction of said charged particles constituting the remainder of the charged particles in said first fraction; d. blocking means located between said low pass filter and said spherical-grid retarding potential filter, said blocking means being disposed relative to the focus of said low pass filter to allow passage of said first fraction to said spherical-grid retarding potential filter while preventing passage of substantially all other charged particles and relative to said spherical-grid retarding potential filter to serve as an inlet means for said first fraction to said spherical-grid retarding potential filter; and e. means for determining the number of charged particles constituting said second fraction.
 14. The energy analyzer of claim 13 wherein said low pass filter is a solid backed electron mirror.
 15. The apparatus of claim 14 further comprising an electrostatically charged prefilter maintained at a second preselected electrical potential level above that at which said low pass filter is maintained for preventing substantially all of the charged particles emanating from said charged particle inlet means with energies above said second preselected electrical potential level from passing to said low pass filter.
 16. A method for analyzing the energy distribution of electrically charged particles emanating from a source with different kinetic energies, comprising the steps of: a reflecting and focusing a first fraction of said charged particles consisting of substantially all of those charged particles incident on a low pass reflecting and focusing filter from such source with kinetic energies below a preselected high level energy limit; b. directing said first fraction of charged particles to a blocking means disposed relative to the focus of the low pass filter to allow passage of said first fraction of charged particles while preventing passage of substantially all other charged particles; c. further directing said first fraction of charged particles which diverge from said blocking means to a retarding field high pAss filter which is constructed so that the equipotential surfaces generated by said high pass filter are substantially normal to the trajectories of the particles within said first fraction at their point of incidence on said high pass filter and which is maintained at a preselected electrical potential level barring passage of those charged particles within said first fraction having kinetic energies below a preselected low leVel energy limit, while permitting passage of a second fraction of charged particles constituting the remainder of the charged particles in said first fraction; and d. determining the number of charged particles constituting said second fraction.
 17. The method of claim 16 wherein said charged particles are subjected to preliminary concentration by passage through an electron condensing lens.
 18. The method of claim 16 wherein said charged particles are subjected to prefiltration, prior to incidence on said low pass filter, to substantially eliminate the effect of parasitic secondary electrons resulting from the impingement of high energy electrons on said electron mirror. 